The present invention relates to cleaning analytical instruments such as Scanning Electron Microscopes (SEM), Scanning Electron Microprobes, Transmission Electron Microscopes (TEM) and other charge particle beam instruments that are subject to contamination problems from hydrocarbons. In particular it is a method and apparatus for cleaning the specimen chamber, specimen stage, and specimen in-situ inside the vacuum system of these instruments.
Electron microscopy is used to detect, measure, and analyze constituents present in very small areas of materials. Contaminants adsorbed on the surface or surface films interacting with the incident electron probe beam can distort the results. Deposits created by the interaction of the probe beam with the surface specimen also may interfere with the probe beam or emitted electrons and x-rays and thus adversely affect accurate analysis. Deposits also add uncertainty to SEM measured line widths for semiconductor device critical dimension metrology.
Another problem is the condensation of pump oils on the windows of the x-ray and electron detectors distorting results. An additional problem of this type is the absorption of low-energy x-rays from Be, C, N, O and F by oil films which can prevent measurement of these elements by X-ray emission spectroscopy.
Contaminants typically are introduced by one of four ways including the specimen, the specimen stage, carried into the chamber by the evacuation system, or are present on the internal components of the instrument. Contaminants introduced from the evacuation system can be reduced by trapping, by purging, or by using cleaner pumps. Once present inside the chamber these contaminants reside on the chamber surfaces, and are typically removed slowly with a low efficiency high vacuum pump.
Inorganic specimens (metals, ceramics, semiconductors, etc.) may carry contaminants into the chamber. These may be part of the specimen, residues from sample preparation techniques or be caused by improper sample handling or storage techniques. In addition, clean surfaces will accumulate a surface film of hydrocarbon scum if left exposed to ordinary room air for any length of time. The sources of these hydrocarbons are most any living thing, organic object, or other source of hydrocarbon vapors in the vicinity. While the part of the films created in these processes dissipate under vacuum conditions, a small amount generally remains on surfaces and is sufficient to cause problems when the specimen is subsequently examined in the analytical instruments listed.
These residues are widely distributed and generally are at low concentrations on the various surfaces. Some of the contaminant molecules become mobile in the vacuum environment. At high vacuum the mean free path of molecules once vaporized is comparable to or longer than the dimensions of the vacuum chamber of these instruments. The contaminants move in the vapor phase from surface to surface in the vacuum environment and are attracted to any focused electron probe beam, forming deposits through an ionization and deposition process. Since these contaminants can travel large distances within the vacuum chamber and over the surface of a specimen, it is important to remove or immobilize these species as much as possible prior to an analysis without disturbing the microstructure of the specimen.
Glow-discharge systems for cleaning SEM chambers use viscous flow vacuum dynamics to carry contaminants from the chamber to the roughing pumps. Most of the current literature and recent patents on glow-discharge cleaning and plasma etch is concerned with the use of these processes in semiconductor production. A variety of gases can be used for etching and cleaning. Gases such as Hydrogen, Argon, Nitrogen, Oxygen, CF4 and gas mixtures such as air and argon/oxygen have successfully been used for glow-discharge cleaning and plasma etching.
Hydrocarbon reactions with oxygen radicals are the dominant reactions in glow discharge cleaning methods using oxygen as a reactant gas. The glow discharge is used to produce oxygen ions that are then transformed into oxygen radicals by subsequent reactions. The oxygen ions are not needed as the reactive species for hydrocarbons. In the absence of nitrogen ions or other reactive species that destroy O radicals, O radicals are long lived and have the ability to do isotropic cleaning on all surfaces in the chamber. CF4 or other fluorine containing gases are sometimes added to oxygen containing plasmas to speed the oxidation of hydrocarbons by performing hydride extraction on the base molecules to make them more susceptible to oxygen attack.
Generally, the normal operation of the SEM is discontinued while the specimen chamber is evacuated of the hydrocarbons using glow-discharge. This results in lost time during the analysis of the specimen. Additionally, vacuum pumps fail to remove the oxidized hydrocarbons from the chamber, thus, distorting the results of the SEM""s specimen analysis. Lastly, at high concentrations and pressures of plasmas, organic samples within the specimen chamber can be damaged or even destroyed.
A method and apparatus for cleaning an analytical instrument while operating the analytical instrument are described. In one embodiment, a method comprises evacuating hydrocarbons from a specimen chamber of an analytical instrument into a plasma chamber via a mesh. The plasma is ignited in the plasma chamber to react with the hydrocarbons.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.